1. Field of the Invention
The present invention relates to a semiconductor device having a circuit comprising a thin film transistor (TFT) that uses a crystalline semiconductor film formed on a substrate (a liquid crystal display device, in particular), and to a method of manufacturing the semiconductor device. The semiconductor device manufactured in accordance with the present invention is specifically a liquid crystal display device represented by an active matrix liquid crystal display device in which a pixel portion and a driver circuit to be placed in the periphery of the pixel portion are formed on the same substrate. The invention also relates to electronic appliances that employ the display device as a display unit.
2. Description of the Related Art
TFTs with a crystalline semiconductor film (typically, a polysilicon film) on an insulating surface as a semiconductor element are used for various integrated circuits at present. The TFTs are used most often as switching elements of a display device. The TFTs having, as an active layer (a semiconductor layer including a channel formation region, a source region, and a drain region), a crystalline semiconductor film, which provides higher mobility than an amorphous semiconductor film, are high in driving performance, and hence used also as elements of a driver circuit. Accordingly, in an active matrix liquid crystal display device, for example, an image circuit for displaying an image and a driver circuit for controlling the image circuit are formed on a single substrate.
In an active matrix liquid crystal display device, integrated circuits such as a pixel circuit for displaying an image, a shift register circuit based on a CMOS circuit, a level shifter circuit, a buffer circuit, and a sampling circuit are all arranged on a single substrate while forming different functional blocks. A liquid crystal display device as above has excellent features including being thin, small-sized, light-weight, and low in power consumption. For that reason, the liquid crystal display device is now used in various scenes; to name a few, as a display unit of a personal computer for space saving and as a display unit of a portable information equipment for obtaining the latest information anytime, any place.
A pixel portion of the liquid crystal display device has a TFT functioning as a switching element (also called a pixel TFT) and a storage capacitor, and is driven by applying a voltage to a liquid crystal. The liquid crystal has to be driven with an alternate current, and a method called frame inversion driving is often employed. The TFT is required to have a characteristic of sufficiently low OFF current (loff: the value of drain current flowing when the TFT is in off-operation). However, OFF current is often high when the TFT is formed of a polysilicon film. A known solution for this problem is the LDD structure with a low concentration impurity region (lightly doped drain) (a structure in which an impurity region having a low concentration is provided between a channel formation region and a source region or a drain region doped with a high concentration of impurity element).
On the other hand, high driving voltage is applied to a buffer circuit and the circuit needs to have a withstand voltage high enough to prevent damage against high voltage. In order to enhance the current driving ability, the ON current value has to be sufficiently high (Ion: the value of drain current flowing when the TFT is in on-operation). Degradation of the ON current value due to hot carriers is effectively prevented by a known structure called the GOLD (gate-drain overlapped LDD) structure in which a gate electrode partially overlaps an LDD region (with a gate insulating film interposed therebetween).
In order to obtain a semiconductor device that meets the required performance, it is necessary to fabricate different TFTs for different circuits. However, increased number of masks are needed to form an LDD structure TFT and a GOLD structure TFT. An increase in number of masks used leads to more manufacture steps, complication of the manufacture process, and reduction in yield.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a semiconductor device, typically, an active matrix liquid crystal display device, in which OFF current of a TFT in a pixel portion is reduced and the reliability of a TFT in a driver circuit is improved (i.e., degradation due to hot carriers is reduced) without increasing the number of masks.
A liquid crystal display device is low in light utilization efficiency, and front light or back light is often used during display in order to improve visibility. The use of front light or back light raises power consumption of its display portion, canceling low power consumption of the liquid crystal display device itself. Accordingly, another object of the present invention is to provide a display device of excellent visibility without increasing the number of manufacture steps.
According to the present invention, there is provided a semiconductor device having a TFT that is formed in a pixel portion and an n-channel TFT and a p-channel TFT that constitute a driver circuit provided in the periphery of the pixel portion, all of the TFTs being formed on the same substrate, characterized in that the n-channel TFT has a second concentration impurity region that partially overlaps a gate electrode, and that the p-channel TFT and the TFT formed in the pixel portion respectively have second concentration impurity regions that do not overlap gate electrodes.
According to the present invention, there is provided a semiconductor device having a TFT that is formed in a pixel portion and an n-channel TFT and a p-channel TFT that constitute a driver circuit provided in the periphery of the pixel portion, all of the TFTs being formed on the same substrate, characterized in that the n-channel TFT has a gate electrode composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of a gate insulating film, the second conductive film contacting the top face of the first conductive film, the first conductive film being longer than the second conductive film in the channel length direction, the first conductive film partially overlapping a second concentration impurity region, and characterized in that the p-channel TFT and the TFT formed in the pixel portion respectively have gate electrodes that do not overlap second concentration impurity regions, the gate electrodes being composed of the first conductive film that contacts the top face of the gate insulating film and the second conductive film that contacts the top face of the first conductive film, the first conductive film and the second conductive film having the same length in the channel length direction.
According to the present invention, there is provided a semiconductor device having a driver circuit that is composed of an n-channel TFT, a first p-channel TFT, and a second p-channel TFT, characterized in that: the n-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a second concentration impurity region; the gate electrode is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film, the second concentration impurity region overlaps the first conductive film with the gate insulating film interposed therebetween; the first p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a fifth concentration impurity region; the channel formation region and the gate electrode of the first p-channel TFT have substantially the same length in the channel length direction; the second p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a fifth concentration impurity region; the gate electrode of the second p-channel TFT is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; and the fifth concentration impurity region of the second p-channel TFT overlaps the first conductive film with the gate insulating film interposed therebetween.
According to the present invention, there is provided a semiconductor device having a driver circuit that is composed of an n-channel TFT, a first p-channel TFT, and a second p-channel TFT, characterized in that: the n-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a second concentration impurity region; the gate electrode is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; the second concentration impurity region overlaps the first conductive film with the gate insulating film interposed therebetween; the first p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, a fifth concentration impurity region and an offset region; the second p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a fifth concentration impurity region; the gate electrode of the second p-channel TFT is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; and the fifth concentration impurity region of the second p-channel TFT overlaps the first conductive film with the gate insulating film interposed therebetween.
According to the present invention, there is provided a semiconductor device having a driver circuit and a pixel portion, the driver circuit being composed of an n-channel TFT, a first p-channel TFT, and a second p-channel TFT, the pixel portion including a TFT and a storage capacitor, characterized in that: the n-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a second concentration impurity region; the gate electrode is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; the second concentration impurity region overlaps the first conductive film with the gate insulating film interposed therebetween; the first p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, a fifth concentration impurity region, and an offset region; the second p-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a fifth concentration impurity region: the gate electrode of the second p-channel TFT is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; the fifth concentration impurity region of the second p-channel TFT overlaps the first conductive film with the gate insulating film interposed therebetween: and the TFT formed in the pixel portion has a semiconductor layer that includes a channel formation region, a source region, a drain region, a second impurity region, and an offset region.
Further, according to the present invention, there is provided a semiconductor device having a driver circuit that is composed of an n-channel TFT, a first p-channel TFT, and a second p-channel TFT, characterized in that: the n-channel TFT has a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the gate insulating film, the semiconductor layer including a channel formation region, a source region, a drain region, and a second concentration impurity region; the gate electrode is composed of a first conductive film and a second conductive film, the first conductive film contacting the top face of the gate insulating film, the second conductive film contacting the top face of the first conductive film; the second concentration impurity region has an Lov region and an Loff region, and the Lov region overlaps the first conductive film with the gate insulating film interposed therebetween whereas the Loff region does not overlap the first conductive film; and the first p-channel TFT and the second p-channel TFT respectively have semiconductor layers, each of the semiconductor layers including a channel formation region, a source region, a drain region, and a fifth concentration impurity region.
In the above present invention, the semiconductor device is characterized in that the gate electrodes of the n-channel TFT, the p-channel TFTs, and the TFT formed in the pixel portion are formed of an element selected from the group consisting of Ta, W, Ti, Mo, Al, and Cu, or formed of an alloy material or a compound material containing any element in the group above as its main ingredient.
In the above present invention, the semiconductor device is characterized in that a plurality of protrusions are formed in the pixel portion; the TFT formed in the pixel portion is electrically connected to a pixel electrode that is uneven; and the uneven portion of the pixel electrode has a radius of curvature of 0.1 to 0.4 xcexcm, and the uneven portion of the pixel electrode is 0.3 to 3 xcexcm tall.